Emergency shutoff valves have been used for decades in flow lines which transport hazardous or combustible fluids. In propane flow lines, for example, emergency shutoff valves are frequently required to insure that a ruptured flow line can be quickly shut in to reduce the likelihood of a fire or explosion. An operator may close such a valve during an emergency by flipping a valve handle, which allows a spring to automatically close the valve. Since the emergency shutoff valve may be positioned in a remote location, it is preferable that the handle may alternatively be activated by pulling on a cable which interconnects the valve handle with an operator access area, thereby again closing the valve. Many emergency shutoff valves may be operated by a fusible link closure mechanism which will automatically close the valve during a fire. Other emergency shutoff valves may include a pneumatic closure mechanism to automatically close the valve if a minimum air pressure to the closure mechanism is not maintained.
One type of emergency shutoff valve utilizes a rotatable stem connected to the valve element. A spring is sized to rotate the stem for automatically closing the valve, but is normally prevented from rotating the stem by a link which serves as a stop to prevent stem rotation. By tripping an actuating handle or by pulling on a cable, the link is moved so that the stem is rotated by the spring to close the valve. Problems with this type of emergency shutoff valve include both unintended closing of the valve and the failure to close the valve when intended. The spring must be sized to provide a sufficient force to close the valve, but a strong spring force may also exert sufficient pressure on the link to trip the valve closed when the valve is bumped or if the flow line is subject to abrupt fluid pressure changes. Also, seals on the valve may dry out and cream a variable drag, so that a valve which remains open for a substantial time period may "freeze" in the open position and not close even after the link is moved.
Another type of emergency shutoff valve utilizes a pivot mechanism to move a stem axially, thereby closing the valve. The pivot mechanism interconnects a valve actuating handle with the valve stem. The pivot mechanism holds the stem in an up position when the valve is open, and rotates on its side to allow the valve to close. A spring for closing the valve when the pivot mechanism rotates may be substantially oversized, since the spring is not creating a rotating force on the swivel body of the pivot mechanism. When the handle rotates the swivel body from the valve open position, the substantial spring force assists continued rotation of the swivel body and automatically closes the valve.
Existing shutoff valves which employ a pivot mechanism to close the valve utilize a fusible link closure mechanism which is separate from the pivot mechanism. A tetherline connects a spring in the closure mechanism to a valve actuating handle. A fusible link normally holds the spring stretched, so that no force is acting on the handle. The link separates when subjected to an elevated temperature indicative of a fire, thereby releasing the stretched spring to pull the tetherline and rotate the handle to close the valve.
Existing shutoff valves with fusible links are expensive to manufacture. In some cases, the fusible link cannot be mounted in its normally intended position relative to the valve. The tetherline which connects the spring with the handle may separate from either the spring or the handle when subjected to the jarring force of a released spring.
The disadvantages of the prior art are overcome by the present invention, and an improved emergency shutoff valve with a fusible link is hereinafter disclosed. The shutoff valve includes a pivot mechanism to close the valve, and a fusible link mechanism is provided for cooperation with the pivot mechanism.